GD5.2 | Magmatic, tectonic and hydrothermal processes at Mid-oceanic ridges and transform faults: new insights from observations and models of the oceanic lithosphere
EDI
Magmatic, tectonic and hydrothermal processes at Mid-oceanic ridges and transform faults: new insights from observations and models of the oceanic lithosphere
Co-organized by GMPV2/TS5
Convener: Marcia Maia | Co-conveners: Eleonora Ficini, Manon Bickert, Florent Szitkar
Orals
| Wed, 26 Apr, 08:30–10:15 (CEST), 10:45–12:30 (CEST)
 
Room -2.47/48
Posters on site
| Attendance Wed, 26 Apr, 14:00–15:45 (CEST)
 
Hall X2
Orals |
Wed, 08:30
Wed, 14:00
Movements across faults allow part of Earth’s surface to move in response to forces driven by tectonic plate motions. Mid-oceanic ridges (MORs) provide the unique opportunity to study two of the three known plate motions: divergence (at the ridge axis) and strike-slip motion along transform faults (crosscutting the ridge axis). Knowledge on active and past processes building and altering the oceanic lithosphere has increased over the past 20 years due to improvements in deep sea technologies and numerical modeling techniques. Yet, several questions remain open, such as the relative role of magmatic, tectonic and hydrothermal processes in the building of the oceanic lithosphere at the ridge axis, especially at slow and ultra-slow spreading ridges and at their intersection with transform faults. Transform faults and their older parts, i.e., the fracture zones, are still poorly studied features. For a long time, they were considered as cold and, for fracture zones, inactive; however, evidences of magmatism have been observed inside both features. Given the complex network of faults existing within these structures, they represent ideal pathways for hydrothermal percolation into the Earth’s lithosphere and may therefore play a significant role in the chemical and the thermal budget of the planet, as well as in the fertilization processes of the oceans in nutrients. This session objective is to share recent knowledge acquired along mid-oceanic ridge axes, transform faults and fracture zones. Works using modern deep-sea high-resolution techniques are especially welcome. The session also welcomes recent developments in thermo-mechanical models, which integrate geophysical and geological data with numerical modeling tools, bridging the gap between observations and numerical models.

Orals: Wed, 26 Apr | Room -2.47/48

Chairpersons: Manon Bickert, Marcia Maia
08:30–08:35
08:35–08:55
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EGU23-9093
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solicited
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Highlight
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On-site presentation
Lars Ruepke, Ingo Grevemeyer, Zhikui Guo, Sibiao Liu, Ming Chen, Jason Morgan, and Colin Devey

Plate tectonics describes oceanic transform faults as conservative strike-slip boundaries, where lithosphere is neither created nor destroyed. Seafloor accreted close to ridge-transform intersections (RTI) has therefore been expected to follow a similar subsidence trend with age as lithosphere that forms away from RTIs. Our recent combined analysis of high-resolution bathymetric data, satellite gravity, and three-dimensional numerical models from transform faults segmenting mid-ocean ridges across the entire spectrum of spreading rates challenges this concept.  One striking observation is that transform faults are systematically deeper than their adjacent fracture zones. Gravity data suggests that the underlying reason may be changes in crustal thickness, with transform valleys having thin and fracture zones ‘normal’ crustal thicknesses. Another observation is that outside corner crust often shows symmetric abyssal hills with intact flat top volcanoes, while the inside corner regions show intense and oblique tectonic deformation. Furthermore, so-called J-shaped ridges, volcanic ridges that bend towards the active transform, show that magmatic accretion occurs predominantly along the spreading axis, ‘feeling’ the rotating stress field only in the direct vicinity of the RTI. While these observations do show some dependence on spreading rate, they can be identified across a wide range of opening rates, suggesting that they are expressions of processes inherent to transform faulting.

In this contribution, we will review these observations before presenting numerical 3-D thermo-tectono-magmatic models designed to elucidate the underlying processes. These models use a dilation term to mimic magmatic accretion and resolve visco-elasto-plastic deformation. The simulations show that the tectonic deformation axis, the axis of plate separation, becomes oblique at depth resulting in extension and crustal thinning within the transform deformation zones. Complementing simulations that account for magmatic accretion and hydrothermal cooling show that a skew can develop between this oblique deformation axis and the axis of magmatic accretion, implying a possible disconnect between the main diking direction and the direction of tectonic deformation. Taken all evidence together, oceanic transform faulting appears to be much more complex than pure strike-slip motion. It shows a surprisingly complex pattern of tectonic faulting and hints at spill-over magmatism at the RTI.  Crustal accretion at ridge transform intersections may therefore be fundamentally different to accretions elsewhere along mid-ocean ridges.

How to cite: Ruepke, L., Grevemeyer, I., Guo, Z., Liu, S., Chen, M., Morgan, J., and Devey, C.: Oceanic transform faults revisited with models and data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9093, https://doi.org/10.5194/egusphere-egu23-9093, 2023.

08:55–09:05
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EGU23-16161
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Highlight
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On-site presentation
Adina E. Pusok, Yuan Li, Richard F. Katz, Tim Davis, and Dave A. May

Observations suggest that the oceanic lithosphere is shaped by dike intrusions and faulting in proportions that depend on the spreading rate (Carbotte et al., 2016). Yet it remains unclear how the interplay between magmatism and faulting during seafloor spreading affects mid-ocean ridge (MOR) axial morphology, fault spacing, and the pattern of abyssal hills (Buck et al., 2005, Huybers et al., 2022). Here we present two-phase flow numerical models of oceanic lithosphere extension that reconcile the nonlinear brittle behaviour of the lithosphere with mantle melting and magma transport through the lithosphere. 

Fast-spreading ridges show symmetric normal faulting and axial highs, while slow-spreading ridges show an asymmetric fault pattern and axial valleys. Previous work has focused on explaining the MOR fault pattern by tectonic or magmatic-induced deformation. In the first scenario, faults result from tectonic stretching of the thin axial lithosphere during amagmatic periods (Forsyth 1992), while in the second scenario, dike-injection may create stresses that activate extensional faults (Carbotte et al., 2016). Current state-of-the-art models (i.e., Buck et al., 2005) use a single-phase formulation for the deformation of oceanic lithosphere in which a prescribed axial dike may accommodate both magmatic and tectonic extension. In these models, the fault pattern depends on M – the fraction of plate separation rate that is accommodated by magmatic dike opening. While M-models are able to explain a number of observations, M represents a simple parameterization of complex fracture dynamics of sills, dikes, and faults. In particular, M-value models neglect fault–dike interaction and other modes of melt transport and emplacement in the lithosphere (Keller et al., 2013). 

Here we build a 2-D oceanic lithosphere extension model that incorporates a new poro- viscoelastic–viscoplastic theory with a free surface (Li et al., in review) to robustly simulate plastic representations of dikes and faults in a two-phase magma/rock system. We hypothesise that magma supply controls the pattern of dike–fault interaction in oceanic extension settings. We present simplified model problems to compare results with those from M-value models. These enable us to address the significance of M in terms of fundamental magma and lithospheric processes. We then focus on development of fault patterns, magma pathways and crustal production at fast-/slow-spreading ridges.

 

References

Buck et al., 2005, Nature, doi:10.1038/nature03358.

Carbotte et al., 2016, Geol. Soc. London, doi:10.1144/SP420.

Forsyth, 1992, Geology, doi:10.1130/0091-7613(1992)020<0027:FEALAN>2.3.CO;2.

Huybers et al., 2022, PNAS, doi:10.1073/pnas.2204761119.

Keller et al., 2013, GJI, doi:10.1093/gji/ggt306.

Li, Y., Pusok, A., Davis, T., May, D., and Katz, R., (in review). Continuum approximation of dyking with a theory for poro-viscoelastic–viscoplastic deformation, GJI.

How to cite: Pusok, A. E., Li, Y., Katz, R. F., Davis, T., and May, D. A.: The role of magma supply in fragmentation of oceanic lithosphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16161, https://doi.org/10.5194/egusphere-egu23-16161, 2023.

09:05–09:15
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EGU23-9688
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On-site presentation
Fernando Martinez and Richard Hey

Mantle melting along mid-ocean ridges occurs in a segmented manner.  Melting and melt extraction are greatest within ridge segment interiors but near segment ends mantle upwelling decreases, cooling increases and melt extraction becomes inefficient.  Owing to the strong influence of water on mantle rheology, these effects have important consequences for the strength of oceanic lithosphere.  Residual mantle formed in ridge segment interiors is melt-depleted and dehydrated forming strong rheological bands.  Near segment ends, however, the formation of low-degree hydrous melts predominates, and these are inefficiently extracted from the mantle.  On solidification, these hydrous melts can re-fertilize surrounding mantle with water due to the high diffusivity of hydrogen in mantle material. This results in weak hydrous bands of mantle material near segment ends.  Thus, segmented mantle melting creates a corresponding segmented oceanic mantle rheological structure that favors the localization of shear deformation in the weak bands near segment ends.  Further strain localization within these weak zones may then facilitate additional weakening processes along discrete narrow transform fault zones. We Illustrate our model with geophysical observations from the Reykjanes Ridge and northern Mid-Atlantic Ridge south of Iceland.  The Reykjanes Ridge is a ~1000 km long linear axis without transform faults.  Rapid propagation of melting anomalies along its linear axis precludes a stable magmatic segmentation as shown by its linear mantle Bouguer anomaly.  Immediately south of the Reykjanes Ridge, the northernmost segments of the Mid-Atlantic Ridge have prominent mantle Bouguer anomaly lows indicating stable cells of segmented mantle melting. Transform and non-transform discontinuities immediately form at the ends of the mantle Bouguer anomaly lows.  This model can be extended to explain the occurrence (or absence) of transform faults over the full range of spreading rates from ultra-slow to ultra-fast ridges.

Reference: Martinez, F., and R. Hey (2022), Mantle melting, lithospheric strength and transform fault stability: Insights from the North Atlantic, Earth and Planetary Science Letters, 579, doi:10.1016/j.epsl.2021.117351.

How to cite: Martinez, F. and Hey, R.: Segmented Mantle Melting, Lithospheric Rheology and Transform Fault Formation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9688, https://doi.org/10.5194/egusphere-egu23-9688, 2023.

09:15–09:25
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EGU23-869
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ECS
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On-site presentation
Sibiao Liu, Zhikui Guo, Lars Rüpke, Jason P. Morgan, Ingo Grevemeyer, and Yu Ren

Gravity signals over the mid-ocean ridge-transform system reflect the distribution of underlying crustal and upper mantle mass anomalies. The gravity measurement, especially ‘residual’ gravity anomalies, relies on the gravitational corrections of both seafloor relief and lithospheric thermal structure. Lithospheric thermal correction typically uses a 1D plate cooling approximation or a 3D passive flow model that assumes isoviscous mantle rheology. As this rheological approximation is oversimplified and physically complex, how sensitive gravity anomalies are to an increasingly complex/accurate approximation for mantle rheology is still unresolved. Here we systematically examine the residual gravity anomaly discrepancies caused by assumptions of different mantle rheologies on 16 natural ridge-transform systems ranging from ultraslow- to fast-spreading. Our calculations show that estimated residual gravity anomalies are significantly lower (e.g., ~21 mGal lower at mid-ocean ridges) in the isoviscous flow models than in the static plate cooling models, primarily due to the effects of lateral heat advection and conduction. When the assumed mantle rheology is changed from uniform viscosity to a non-Newtonian viscosity with brittle weakening in cooler (faulting) regions, the mantle upwelling intensifies and local near-surface temperature generally increases, resulting in an increase in the residual anomaly. This increase is distributed uniformly along the ultraslow-and slow-spreading ridge axes, but is concentrated along transform faults at intermediate- and fast-spreading ridges. The amount of the rheology-induced gravity difference is most closely linked to transform age offset instead of spreading rate or transform offset length alone. Our analysis reveals that oceanic transform faults exhibit higher gravity anomalies than adjacent fracture zones, which may reflect thinner crust in the transform deformation zone.

How to cite: Liu, S., Guo, Z., Rüpke, L., Morgan, J. P., Grevemeyer, I., and Ren, Y.: Gravity signature in the mid-ocean ridge-transform system: Insights from deep mantle rheology and shallow crustal structure, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-869, https://doi.org/10.5194/egusphere-egu23-869, 2023.

09:25–09:35
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EGU23-9241
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ECS
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On-site presentation
Katharina A. Unger Moreno, Colin W. Devey, Lars Rüpke, Anouk Beniest, Thor H. Hansteen, and Ingo Grevemeyer

Recent studies on oceanic transform faults, one of the three fundamental types of plate boundaries, has suggested that they may not be purely conservative features and that the crust formed adjacent to them (on the "inside corners" of the ridge-transform intersection) may differ in structure and composition significantly from outside-corner crust. Here we present a geological map of the Oceanographer Transform (Atlantic Ocean, southwest of the Azores) created by combining an interpretation of multibeam bathymetry, rock sampling and seafloor visual observations. We find that outside- and inside-corner crust at the ridge transform intersection have distinctive morphologies and petrography: the outside corner shows rough seafloor, from which only pillow basalts are recovered, extending all the way to the fracture zone. The inside corners, in contrast, are characterized by both rough, basaltic seafloor and regions that are much smoother, from which serpentinized peridotite are often recovered. The width of the inside-corner region showing this variable morphology, bathymetry and petrography seems to vary over time from 10 to 25 km. In two places, oceanic core complex crust is recognized close to the transform in this inside-corner region. We emphasize that plate production at the inside corner appears to occur via a variety of magmatic and amagmatic processes.

How to cite: Unger Moreno, K. A., Devey, C. W., Rüpke, L., Beniest, A., Hansteen, T. H., and Grevemeyer, I.: Geological overview of the Oceanographer Transform Fault, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9241, https://doi.org/10.5194/egusphere-egu23-9241, 2023.

09:35–09:45
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EGU23-2737
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ECS
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On-site presentation
Isabel Kremin, Zhikui Guo, and Lars Rüpke

The significant discrepancy between the observed conductive heat flow and predictions by thermal models for oceanic lithosphere younger than 50 Ma is generally interpreted to result from hydrothermal circulation between basement outcrops. Numerical simulations of fluid flow between such outcrops performed in previous studies revealed that establishing horizontal pressure gradients to sustain a hydrothermal siphon requires high aquifer permeabilities and a contrast in the outcrops’ transmittance, which is the product of the outcrop permeability and the area of outcrop exposure. However, most previous studies focused on the model parameters needed to sustain a hydrothermal siphon, while the physical processes that create the horizontal pressure gradients in the first place remain poorly constrained.

In order to shed more light on the physics behind outcrop-to-outcrop flow, a simple synthetic 2D model of two outcrops connected by a permeable aquifer was set up. Fluid flow modelling was done by using hydrothermalFoam, a hydrothermal transport model, that is based on the open-source C++ computational fluid dynamics toolbox OpenFOAM. Our initial simulations focus on variations of the permeability of the outcrops and the aquifer. The results reveal two key points that are essential to generate a flow: First, the outcrops permeability has a fundamental effect on its average pressure. High permeabilities lead to a rather "cold" hydrostatic pressure regime with lower temperatures and hence higher average pressures. Lower outcrop permeabilities are accompanied with a rather "warm" hydrostatic pressure regime characterized by higher temperatures and lower average pressures. Secondly, fluid convection in the aquifer is necessary to establish a siphon flow. Therefore, the aquifer permeability must be sufficiently high to overcome Darcy resistance and yet low enough to prevent the flow from being solely diffusive.

How to cite: Kremin, I., Guo, Z., and Rüpke, L.: The effect of permeability on the pressure regime in 2D outcrop-to-outcrop submarine hydrothermal flow models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2737, https://doi.org/10.5194/egusphere-egu23-2737, 2023.

09:45–09:55
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EGU23-14327
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On-site presentation
Soumya Bohidar, Wayne Crawford, and Mathilde Cannat

Lucky Strike volcano is the central edifice of the Lucky Strike segment, Mid-Atlantic Ridge. Its summit overlies an axial magma chamber (AMC), 3-3.8 km beneath the seafloor, and hosts one of the largest known deep-sea hydrothermal fields. Local seismicity beneath the hydrothermal field has been monitored since 2007 as a part of the EMSO (European Multidisciplinary Seafloor and water column Observatory)-Azores observatory by 5 OBSs with yearly redeployments. In a 12-year (2007-2019) earthquake catalog (noncontinuous), we observe continuous low magnitude seismicity (ML ~ -1 to 0), focused mainly 0.5-2 km above the AMC, suggesting that thermal contraction of rocks, possibly combined to deformation induced by volume changes during hydrothermal alteration, at the base of a single limb along-axis hydrothermal cell is the primary source of this seismicity. We thus interpret the seismicity clusters, with horizontal extent 1200 to 1800 m2, as zones of enhanced heat extraction, in the lower part of the hydrothermal downflow zone.

We present the evolution of this hydrothermally-induced seismicity over the 12 years of the catalog. We observe three lateral 400-800 m shifts of the main seismicity clusters. The first and second shifts are small and could be explained by a fortuitous combination of network-based biases, picking error and/or change in the shallow seismic velocity structure of the volcano. The third shift, occurring during a catalog gap between June 2013 and April 2015, is ~800 m eastward and corresponds to a change in the seismicity distribution from a patch above the AMC to a vertical pipe-like pattern, indicating a real change in the hydrothermal circulation. We propose that this shift is driven by recent magmatic injections above the AMC, and/or to the opening of new tectonic cracks, enhancing local permeability and allowing for more efficient cooling above the shallower region of the AMC roof.

We also observe three Higher Seismic Activity (HSA seismic rate > 18 events/week) periods: April-June 2009, August-September 2015, and April-May 2016. The 2009 HSA period was the most intense: it lasted ~13 weeks, starting with a relatively higher magnitude event (ML = 1.7), and culminating in June after another higher magnitude (ML = 1.8) event. Most of the events clustered 0 to 1 km above the AMC reflector, with a few deeper events (down to only 800 m below the AMC reflector) during the culmination period. Although we do not have focal mechanisms to test this hypothesis, we propose that this HSA period resulted from tectonic events opening enhanced local permeability channels for downgoing hydrothermal fluids, and leading to higher heat extraction by the hydrothermal system.

How to cite: Bohidar, S., Crawford, W., and Cannat, M.: Seismic constraints on the evolution of hydrothermal circulation beneath Lucky Strike volcano, Mid-Atlantic Ridge, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14327, https://doi.org/10.5194/egusphere-egu23-14327, 2023.

09:55–10:05
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EGU23-2230
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ECS
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Highlight
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On-site presentation
Matthias Pilot, Marie Eide Lien, Vera Schlindwein, Lars Ottemoeller, and Thibaut Barreyre

In recent years hydrothermal vent systems were found in unexpectedly high abundance along ultraslow spreading ridges, despite their overall decreased magma supply. Thin oceanic crust and resulting shallow heat sources can drive hydrothermal fluid circulation and detachment faults can act as fluid pathways, resulting in e.g., serpentinization of the oceanic crust. So far, no long-term recording of seismicity around hydrothermal vent systems along ultraslow spreading ridges have been reported. Here, we present results from a ~1-year local Ocean Bottom Seismometer deployment between 2019 - 2020 at Loki’s Castle hydrothermal vent field (LCVF) along the Arctic Mid Ocean Ridge. LCVF is located at a water depth of ~2500m on top of the axial volcanic ridge (AVR) at the Mohn-Knipovich Ridge bend where spreading is highly asymmetric from west to east.

For the processing we use a combination of an automatic event detection algorithm (Lassie), a deep-learning phase picking model (PhaseNet) and partial manual re-evaluation of phase picks. Additionally, selected clusters of events are cross-correlated and relocated using hypoDD. The resulting earthquake catalogue consists of a total of 12368 events with 6719 manually re-evaluated and 5649 automatically picked events.

From the results we see that most of the plate divergence at the Mohn-Knipovich Ridge bend is accommodated by a young detachment fault west of the AVR. Most of the seismicity occurs between depths of ~2-8km in a bended band that steepens up to 70° with depth and follows the local topography. However, the described detachment fault differs from reported mature detachment faults at the Mid-Atlantic Ridge or Southwest Indian Ridge. Within the footwall we observe episodical, clustered seismicity with extensional faulting mechanisms, indicating that the detachment could be cross-cut by normal faults. Along strike, the seismicity of the fault plane appears highly heterogeneous, with the central part showing only sparse seismicity at depths below 3km while other segments show episodical shallow seismicity. Towards LCVF seismicity below the AVR increases and the maximum depth of earthquakes shallows by about ~2km. This could indicate the presence of a shallow heat source below LCVF as a driving factor for the hydrothermal circulation.

How to cite: Pilot, M., Lien, M. E., Schlindwein, V., Ottemoeller, L., and Barreyre, T.: Highly Asymmetric Seismicity in a System of Tectonic Extension and Hydrothermal Venting at the Mohn-Knipovich Ridge Bend, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2230, https://doi.org/10.5194/egusphere-egu23-2230, 2023.

10:05–10:15
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EGU23-1567
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ECS
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Highlight
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Virtual presentation
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Viacheslav Bogoliubskii, Evgeny Dubinin, and Andrey Grokholsky

Rift zones of Iceland large igneous province (LIP) have complicated interior geometric pattern expressing in several parallel extension centers. It significantly differs from adjacent Reykjanes (RR) and Kolbeinsey (KR) mid-oceanic ridges (MOR) that only have small overlappings between separate neovolcanic centers. At small scale, rift zones connect with each other by broad transform zones with distributed strain pattern instead of typical narrow transform faults. Those transform zones have very different structure varying from simple book-shelf fault zones of South Iceland seismic zone to sophisticated system of magmatic and amagmatic structures of Tjörnes transform zone. The whole system drastically differs from typical structure and geometry of ultra-slow MOR. Iceland rift zone evolution commenced at 25 Ma and strongly influenced by thermal pulses of Iceland plume each 6-7 My and slightly asymmetric spreading. Another challenge of this region lies in asymmetric thermal influence of Icelandic plume. RR is affected by plume at distance of at least 800 km, whereas Kolbeinsey ridge at distance of ca. 600 km. To reveal the ridge-plume interaction through Iceland evolution and possible causes of Icelandic plume influence asymmetry we used a method of physical modelling. The extending setting comprises mineral oils mixture that have numerical resemblance with oceanic crust in density, shear modulus and thickness. Two-layered model have elastic bottom layer, brittle top one and local heating source (LHS) corresponding to Icelandic plume pulses. The first experiment type configuration includes two sections corresponding to RR and KR. At their joint, the LHS melts the modelling lithosphere creating analogue of LIP. The LHS periodically switched on and transported to another position, which is similar to plume pulses in asymmetric spreading conditions. The general pattern of each cycle is as following. Initially within LIP two rift branches propagate to each other forming an overlapping. A block between two rift branches rotates as horizontally as vertically. These blocks express in Iceland topography as uplifted peninsulas of its northwestern part. In some time, overlapping transforms to oblique transfer zone and rift zones change their structure of several extension centers to one-axis structure and have direct connection. Then new plume pulse rejuvenates the cycle. If incipient offset between rift branches is quite small, then overlapping structure passes to oblique transform zone with several extension centers and small overlappings. Thermal pulses of less volumes have considerable influence as well, but current data cannot permit to correctly them. As a result, we created a conceptual model of Iceland rift zones evolution also using data of other researchers. The second model had the same initial configuration, but thermal pulses extend downward to modelling Reykjanes ridge. This migration caused by density heterogeneities of upper layers due to deep thermal differences. The resulting geometry is very similar to natural one. There are different segmentation pattern at both spreading ridges and some rift zones. Developed transform zones confine rotating blocks and have structure varying from book-shelf fault zone to overlapping as in nature. We infer that modeled asymmetry and origination can reflect the natural ones.

How to cite: Bogoliubskii, V., Dubinin, E., and Grokholsky, A.: Evolution of Icelandic rift zones geometry as result of MOR-plume interaction, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1567, https://doi.org/10.5194/egusphere-egu23-1567, 2023.

Coffee break
Chairpersons: Eleonora Ficini, Marcia Maia
10:45–11:05
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EGU23-17478
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solicited
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Highlight
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On-site presentation
Ana P. Jesus

On-going work on the Samail Ophiolite Volcanogenic Massive Sulfide (VMS) deposits and Oman Drilling Project (OmanDP) drill cores provide insights on sulfur and metal cycling during hydrothermal alteration and critical differences between ophiolitic and modern oceanic crust.

The volcanic section is pervasively overprinted by low-T oceanic metasomatism leading to variably depleted sulfur in sulfide (TSsulf) concentrations reflecting leaching and oxidation of magmatic sulfides. Secondary sulfides incorporated mostly basaltic sulfur with minor sulfur addition via open-system bacterial sulfate reduction (BSR).

The sheeted dyke-gabbro transition (OmanDP GT3 drillhole) records a change from BSR open-system processes (d34S>-12.8‰) towards addition of heavy hydrothermal sulfur via thermochemical sulfate reduction ~4km above the Moho Transition Zone-MTZ. Downward progression from d34S=+13.6‰ to ~MORB values suggest decreasing water/rock ratios during hydrothermal alteration. Here, near complete recrystallization under greenschist/amphibolitic facies conditions (no magmatic sulfides), coupled with strong sulfur (TSsulf>2 ppm) and copper leaching (>1 ppm), document the high-T reaction zone of the hydrothermal system overlying the axial melt lens, where S and metals are sourced to form VMS deposits. Although multiple sulfur isotope systematics for Oman VMS ores indicates a deep S-source within the range of GT3 reaction zone (d34S ~4‰), REE patterns and trace metal endowments in the ores suggest that the footwall lavas are also a source of metals, in addition to those leached from the deep reaction zone. Crucially, metal leaching and S-isotopic shifts are far more extensive than those reported on in-situ oceanic crust, implying a net addition of seawater-S ~30% to the upper crustal section.

Differences between in-situ and ophiolitic lower crustal sections are seemingly less pronounced: the foliated and layered gabbros (GT2-GT1 drillholes) preserve small S-isotopic shifts relative to MORB, implying that formation of secondary sulfides involved minor S-seawater input (~7%) and mostly redistribution of magmatic-S. Wide fault zones of convincing oceanic origin preserve sulfates with composition similar to Cretaceous seawater (d34S~+18‰) supporting the role of focused fluid flow corridors during deep crustal cooling. TSsulf and Cu+Ni concentrations increase throughout the lower crust while strong Cu+S leaching characterize tectonized and low-T hydrothermally overprinted domains. Above the MTZ, the primitive layered gabbros and intercalated ultramafics (CM1 drillhole- Sequence SI) record metal and TSsulf enrichments related with magmatic sulfide saturation/segregation from mantle melts upon entering the crust. Incompatible element rich pegmatoidal dikelets crosscutting SI include late, high-fS2 sulfides formed during low-T BSR (δ34S>-25.8‰).

The MTZ comprises 90m of fully serpentinised dunite (SII) underlain by dunite with rodingitized gabbro (SIII). The SII-dunites show vanishing TSsulf and Cu concentrations, consistent with desulfurization producing alloy-bearing mineral assemblages formed during extremely low fS2-fO2 conditions, typical of early serpentinization stages. The dunites mark the onset of increasing S-isotopic shifts towards the SIII-rodingites The occurrence of both sulfides (δ34S=+1.4, +56.9‰) and sulfates (δ34SSO4=+19.4, +36.5‰) with δ34S>>Cretaceous seawater sulfate can be explained by input of fluids at the top of SII-dunites which composition progressed towards extreme heavy values during closed-system, multi-staged evolution.

AJ acknowledges WWU International Visiting Scholars and EU-H2020 Marie Sklodowska-Curie #894599 Fellowships, and FCT I.P./MCTES PIDDAC–UIDB/50019/2020- IDL.

How to cite: Jesus, A. P.: Sulfur and metal fluxes in the oceanic crust: the Samail  ophiolite as proxy for fast spreading ridges., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17478, https://doi.org/10.5194/egusphere-egu23-17478, 2023.

11:05–11:15
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EGU23-11852
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Highlight
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On-site presentation
Jürgen Koepke, Dieter Garbe-Schönberg, Dominik Mock, and Sven Merseburger

Based on a newly established profile through fast-spreading oceanic crust of the Oman ophiolite and on cores drilled within the ICDP Oman Drilling Project (OmanDP), we present here the results of 12 years research, focusing on the nature of the magmatic accretion of the deep crust beneath fast-spreading mid-ocean ridges. We established a 5 km long profile through the whole plutonic crust of the Oman ophiolite by systematic outcrop sampling in the Wadi Gideah (Wadi Tayin Block near Ibra), providing the reference frame for the 300 to 400 m long OmanDP drill cores GT1 and GT2 (lower crust, mid-crust), as well as CM1 and CM2 (crust-mantle boundary) drilled into the same area.
The results allow implication on the mechanism of accretion of fast-spreading lower oceanic crust. Depth profiles on bulk rock and mineral compositions, crystallization temperature and microstructures combined with petrological modeling reveal insights into the mode of magmatic formation of fast-spreading lower oceanic crust, implying a hybrid accretion mechanism. The lower 2/3 of the crust (mainly layered gabbros) was formed via the injection of melt sills and in situ crystallization. Here, upward moving fractionated melts mixed with more primitive melts through melt replenishments, resulting in an upward differentiation trend. Since the fraction of crystallization is only small, upmoving melts could easily transport the latent heat produced by deep crystallization upward. The upper third of the gabbroic crust is significantly more differentiated, in accord with a model of downward differentiation of a parental melt originated from the axial melt lens sandwiched between the gabbroic crust and the sheeted dike complex. While the 5 km long profile shed light on the overall magmatic accretion process, the Oman DP drill cores showing ~ 100% recovery allowing high density sampling provide incredible details on the magmatic accretion process. Examples are the identification of individual melt sills from which the layered gabbro section has been formed (drill core GT1) or the detailed observation of olivine accumulation at the base of the crust (drill cores CM1/CM2).

How to cite: Koepke, J., Garbe-Schönberg, D., Mock, D., and Merseburger, S.: Accretion of fast-spreading oceanic crust: benefits from large-scale sampling in the Oman ophiolite in combination with cores drilled by the ICDP Oman Drilling Project, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11852, https://doi.org/10.5194/egusphere-egu23-11852, 2023.

11:15–11:25
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EGU23-742
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ECS
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On-site presentation
Léa Grenet, Marcia Maia, Cédric Hamelin, Anne Briais, and Daniele Brunelli

At mid-oceanic ridges, mantle temperature and magma supply influence the structure of the neo-volcanic zone. Due to the large Romanche offset, a strong “cold edge” effect is present at its eastern intersection with the Mid-Atlantic Ridge. This effect decreases with the distance from the transform fault, making this region an ideal area to study the impact of the thermal gradient on the architecture of the neo-volcanic zone. We analyzed seafloor videos and photos from submersible dives, as well as bathymetry and backscatter data collected during the SMARTIES cruise (2019), from the Ridge-Transform Intersection (RTI) to approximately 80 km to the south of it. We produced maps at local and regional scales and quantified the morphology of volcanoes (height, diameter, height/diameter ratio, volume and surface). Visual observations have showed that the seafloor is mainly made up of pillows or elongated pillows and rare massive lava flows. Within 30 km of the RTI, the neo-volcanic area is characterized by clusters of volcanoes, affected by faults trending N120-130° E, oblique to the extension and to the transform fault orientation, and by faults trending E-W. At 30 km to 50 km from the RTI, the Central segment displays a robust volcanic ridge oriented N150°E built by a pilling of volcanoes and narrow ridges. Its eastern and southern parts are old and characterized by oblique faults and narrow ridges (N130-140°E), while the northwest portion is more recent, faults and ridges are almost normal to the extension. The southernmost segment, located at 80 km from the RTI, is orthogonal to the spreading direction and asymmetric, bounded at the west by a detachment fault. Recent volcanic edifices were observed from the center of the segment to the base of the detachment. Our observations suggest that the neo-volcanic area is fed by more and more magma from north to south. This increase in magma supply is marked by a more structured volcanic axis, volcanoes that are more voluminous and a change in the orientation of the segments and faults. Changes in the orientation of faults and off-axis abyssal hills also reveal variations in the magmatic supply over time.

How to cite: Grenet, L., Maia, M., Hamelin, C., Briais, A., and Brunelli, D.: Construction of the neo-volcanic area of a slow-spreading ridge in a cold mantle context: Mid-Atlantic Ridge Eastern Intersection with Romanche Transform Fault, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-742, https://doi.org/10.5194/egusphere-egu23-742, 2023.

11:25–11:35
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EGU23-11070
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On-site presentation
Adrien Moulin and Sigurjón Jónsson

Lithospheric inheritance is known to strongly influence the spatial and temporal patterns of continental deformation in all geodynamic contexts, emphasizing the role of rheological feedbacks between time-spaced geodynamic events. In principle, the transition from continental rifting to sea-floor spreading at diverging plate boundaries marks a threshold beyond which these long-term feedbacks no longer apply. This is because sea-floor spreading is accompanied by the creation of new lithosphere from melting and cooling of the underlying and uprising mantle, which should make lithospheric inheritance negligible at oceanic plate boundaries. However, whether and how lithospheric inheritance continues to affect oceanic plate boundary processes after the continental rifting to sea-floor spreading transition is reached has so far not been explored in detail.

As a young oceanic rift that broke up the Arabia-Nubia Shield and its mosaic of Proterozoic accreted blocks, the Red Sea (RS) represents an ideal case to study these specific lithospheric inheritance effects. We performed a quantitative morpho-structural analysis designed to track along-axis variations of the magmato-structural architecture of the RS plate boundary and to explore its relationships with the inherited structures of the rifted continental plates. Specifically, faults and sea-floor morphology have been mapped over the post-5.3Ma extent of oceanic crust from Global Multiresolution Synthesis (including multibeam surveys) bathymetry. The structural and magmatic patterns have then been extracted by quantifying four metrics: the axial depth, the slope of the central-trough flanks, the proportion of exposed volcanic sea-floor, and the distribution of normal-fault offsets.

This analysis reveals that anomalously deep segments bounded by steeper-than-average flanks bound the central RS in the North and South. Furthermore, it shows that this specific axial topography occurs where the structural pattern locally switches from regularly-spaced and moderate-displacement (~400m) normal faults to one dominant large-displacement (~1200m) fault as well as coinciding with a lower proportion of volcanic sea-floor (15-20% versus 70% on average along the rest of the axis). This distinct magmato-structural signature is commonly interpreted to reflect a decreased fraction of plate separation accommodated magmatically along slow and ultra-slow spreading ridges, in agreement with tectono-magmatic interaction models: individual faults that form near the axis remain active longer and accumulate more displacement when this fraction decreases. On the other hand, a decreased magma input would result in a thinner crust, and thus isostatically account for the anomalous depth of these segments.

The location of these two magma-starved segments appears unrelated to variations in spreading rate or to the segmentation of the RS axis, but stands in the prolongation of two major Proterozoic suture zones within the Arabia-Nubia Shield. On the Arabian side, both of these two inherited structures coincide with a rise of the lithosphere-asthenosphere boundary (LAB) as mapped from S-to-P receiver functions. We therefore propose that on-axis magma starving results from local outward spreading of the upper-mantle upwelling, in turn driven by its off-axis channeling along the LAB topographic highs. Thereby, heat and eventually melts would be transferred from beneath the axis to beneath the onshore suture zones, possibly fueling the Plio-Pleistocene volcanic activity observed there.

How to cite: Moulin, A. and Jónsson, S.: Lithospheric inheritance controls on early sea-floor spreading: new insights from magmato-structural patterns along the Red Sea axis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11070, https://doi.org/10.5194/egusphere-egu23-11070, 2023.

11:35–11:45
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EGU23-12073
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On-site presentation
Leila Mezri, Javier García-Pintado, Marta Pérez-Gussinyé, Zhonglan Liu, Wolfgang Bach, and Mathilde Cannat

At ultra-slow ridges, tectonics, hydrothermalism, serpentinization and magmatism interact to build the oceanic crust. How this heterogenous crust forms and relates to faulting remains poorly understood, but is key for elucidating hydrothermal flow patterns and their implications for ocean-lithosphere element exchange. Along the melt-poor Southwest Indian Ridge (SWIR) at 64°30' East, crustal thickness varies across the ridge strike, with crustal thickening attributed to serpentinization extending downward along detachment faults, DFs. This observation calls into question the commonly assumed relationship between local crustal thickening and magma-supply increase. Here we use 2D numerical models to analyze how coupled tectonics, mantle melting, magma emplacement and serpentinization interact. Our model includes hydrothermal cooling, ocean loading, and the oceanic crust density. We reproduce the observed bathymetry at SWIR, 64°30'E, which is shaped by alternating DFs formed in flip-flop mode. Our results show that the offset and duration of DFs are controlled by ocean loading and crustal density. Importantly, shallow faulting and deeper mantle flow are coupled: long-lived DFs result in relatively slower mantle upwelling, lower melt supply, but crustal thickening due to deeper serpentinization, ~5 km, consistent with the observed thick ultramafic crust in nature. In between alternating DFs, mantle upwelling is faster, melt supply higher, and serpentinization shallower, < 2km. Since magmatic crustal thickness is overall very small, 1.5-2 km, changes in faulting-induced serpentinisation depth, are the main cause for observed variations in crustal thickness, 2-7 km. We conclude that, at melt-poor ridges, tectonics controls both crustal thickness variations and melt supply oscillations.

How to cite: Mezri, L., García-Pintado, J., Pérez-Gussinyé, M., Liu, Z., Bach, W., and Cannat, M.: Tectonics controls on melt production and crustal architecture during nearly amagmatic seafloor spreading, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12073, https://doi.org/10.5194/egusphere-egu23-12073, 2023.

11:45–11:55
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EGU23-6200
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ECS
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On-site presentation
Souradeep Mahato and Mathilde Cannat

The eastern Southwest Indian Ridge (SWIR) is a melt-poor end-member region of the global MOR system. The available magma focuses to axial volcanoes, leaving >50 km-wide, nearly amagmatic ridge sections, where seafloor spreading occurs via large offset detachment faults. We present map to outcrop scale observations of the deformation associated to one of these detachments, in the 64°35'E region of the SWIR. This active detachment fault presently has a horizontal offset of ~4 km (Cannat et al., 2019), and accommodates nearly all the plate divergence (14 km/million year; Patriat and Segoufin, 1988). We focus on the lower slopes of the footwall, where this active fault currently emerges at an angle of ~35°. The emergence is traceable across a length of ~20 km on side-scan sonar and shipboard bathymetry data. It locally shows undulations at a wavelength of ~1-4 km. High-resolution bathymetry at and near the emergence area shows two morphological domains. In one domain, the exhumed fault surface bears distinct corrugations that trend at an 18° to 33° angle to the spreading direction, extends up to 300 m, are spaced by ~15-300 m, and are ~1 m to ~40 m in amplitude. In the other domain, the exhumed fault is not corrugated. Remotely operated vehicle (ROV) dive observations at the outcrop scale show discrete planar fault planes and brecciated and fractured rock forming the top ~1-4 meters of the corrugated exposures. In contrast, the non-corrugated fault exposures show up to ~8 m of gouge-bearing micro-brecciated domains, including several up to 1 m thick horizons of semi-brittle sheared serpentinites. Dive observations further suggest that: (1) there are several sigmoidal intercalations of such gouge-bearing horizons forming the upper few tens of meters of the non-corrugated fault zone, and (2) the horizons of sheared serpentine originated as brittle cracks that served as hydrous fluid pathways into the fault damage zone. We propose: (1) that the absence of corrugations is related to the overall weaker semi-brittle rheology of the emerging fault in this domain, compared to the purely brittle corrugated domain; and (2) that the two domains represent damage developed in distinct conditions of temperature and hydrothermal fluid availability. At the broader map scale, the non-corrugated domain to the east emerges about 1.2 km farther south than the corrugated domain, and the trace of emergence thus draws an indentation between the two domains. Given the ~35° fault emergence angle in the two domains, we infer that their across-fault distance is ~650 m. The detachment damage zone may thus be at least that broad, and comprised of distinct, probably anastomosing domains of more localized deformation, which would preferentially be exposed at the seafloor. This damage zone anatomy would be consistent with seismic refraction observations (Momoh et al., 2017) in the area.

How to cite: Mahato, S. and Cannat, M.: Anatomy of a detachment fault damage zone at a nearly amagmatic mid-ocean ridge: observations from outcrop to map scale, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6200, https://doi.org/10.5194/egusphere-egu23-6200, 2023.

11:55–12:05
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EGU23-15377
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ECS
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On-site presentation
Utpalendu Haldar, Simontini Sensarma, and Ramananda Chakrabarti

Hydrothermal alteration of seafloor basalts alters its elemental and isotopic composition. Studies on dredged basalts and ophiolite sequences using stable O, K, and radiogenic Sr [1,2,3] isotopes have documented the effect of seafloor alteration on such lithologies. Experimental studies of basalt-seawater interaction have also demonstrated exchange of Sr isotopic signatures between these two phases [4] while limited data for altered oceanic crust suggests incorporation of heavier Sr isotopes [5]. To further our understanding of the behaviour of Sr during seafloor alteration in natural settings, we measured 87Sr/86Sr and δ88/86Sr in a suite of variably altered lithologies from Hess Deep Rift (HDR), which include basalt, norite, gabbro and troctolite.

Radiogenic Sr (87Sr/86Sr) was measured using TIMS (Thermo Scientific, Triton Plus), using internal normalization while stable Sr isotopes (δ88/86Sr, reported relative to NIST SRM 987) were measured using a double spike (84Sr-87Sr) TIMS technique, both at the Centre for Earth Sciences, IISc, Bangalore. The δ88/86Sr values of the HDR samples (0.308-0.810 ‰) are higher than the bulk silicate Earth (BSE) value (0.27 + 0.05 ‰) [6]; some samples show δ88/86Sr values higher than modern-day seawater value (0.386 ‰) [e.g., 7]. The 87Sr/86Sr varies from ~0.703 in unaltered samples to ~0.709 in altered samples, the latter close to the modern-day seawater value. Overall, our data suggests incorporation of heavier isotopes of Sr in altered oceanic crustal samples; the heavier than seawater δ88/86Sr values observed in some samples reflect formation of new mineral phases, consistent with high δ88/86Sr observed in anhydrite formed in laboratory experiments of basalt-seawater interaction[4].

[1]. Lamphere et al. (1981) Journal of Geophysical Research: Solid Earth86(B4), pp.2709-2720; [2]. McCulloch et al. (1981) Journal of Geophysical Research: Solid Earth86(B4), pp.2721-2735; [3]. Parendo et al. (2017) Proceedings of the National Academy of Sciences114(8), pp.1827-1831; [4]. Voigt et al. (2018) Geochimica et Cosmochimica Acta240, pp.131-151; [5] Klaver et al. (2020) Geochimica et Cosmochimica Acta288, pp.101-119; [6] Moynier et al. (2010) Earth and Planetary Science Letters300(3-4), pp.359-366; [7]. Ganguly and Chakrabarti 92022) Journal of Analytical Atomic Spectrometry37(10), pp.1961-1971.

How to cite: Haldar, U., Sensarma, S., and Chakrabarti, R.: Significant variability in 87Sr/86Sr and d88/86Sr in Hess Deep Rift lithologies due to hydrothermal alteration., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15377, https://doi.org/10.5194/egusphere-egu23-15377, 2023.

12:05–12:15
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EGU23-12754
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Highlight
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On-site presentation
A new model for the evolution of oceanic transform faults based on 3D Broadband Seismic observations from São Tomé and Príncipe in the eastern Gulf of Guinea.
(withdrawn)
Myron Thomas, Christian Heine, Jimmy van Itterbeeck, Ilya Ostanin, Andrey Seregin, Michael Spaak, Tamara Morales, and Tess Oude Essink
12:15–12:25
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EGU23-3138
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ECS
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On-site presentation
Peter Haas, Myron Thomas, Christian Heine, Jörg Ebbing, Andrey Seregin, and Jimmy van Itterbeeck

The Eastern Gulf of Guinea hosts several buried Cretaceous-aged oceanic fracture zones. 3D broadband seismic data acquired offshore São Tomé and Príncipe revealed a complex crustal architecture. Mapped oceanic fracture zones show low-angle reflectors that detach onto or eventually cross through the Moho boundary, overlain by strong reflectors that are interpreted as transform process related extrusive lava flows. Here, we use a high resolution shipborne free-air gravity and total field intensity magnetic data set to reassess whether previously defined seismic models of the crust are in conformity with potential field data. The study area is located offshore São Tomé with a size of c. 150x150 km. Using the software IGMAS+, we model the gravity and magnetic properties of the crust (i.e. density and susceptibility) in 3D. Long record length seismic sections plus mapped seismic horizons, which include bathymetry, sediments, upper and lower crust, are used as constraints. While the general trend of the free-air anomaly can be explained within a range of expected crustal densities, the magnetic field anomaly reflects high residuals that are predominantly oriented parallel to the transform faults. This indicates that gravity and magnetic data cannot be explained by the same simple source geometry. Therefore, we first perform sensitivity tests to isolate the source of the residual magnetic anomaly, followed by a structural analysis along the transform faults with special emphasis to the extrusive lava flows in the crustal domain. Our final model reconciles seismic horizons and potential field data and will stimulate a discussion on the architecture and evolution of transform faults and their signatures in different data sets.    

How to cite: Haas, P., Thomas, M., Heine, C., Ebbing, J., Seregin, A., and van Itterbeeck, J.: Oceanic transform faults offshore São Tomé and Príncipe highlighted by integrated density and magnetic modeling of the crust, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3138, https://doi.org/10.5194/egusphere-egu23-3138, 2023.

12:25–12:30

Posters on site: Wed, 26 Apr, 14:00–15:45 | Hall X2

Chairpersons: Eleonora Ficini, Manon Bickert
X2.126
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EGU23-1879
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Wojciech Czuba, Rolf Mjelde, Yoshio Murai, and Tomasz Janik

The structure of the oceanic crust generated by the ultraslow-spreading mid-ocean Knipovich Ridge still remains relatively uninvestigated compared to the other North Atlantic spreading ridges further south. The complexity of the Knipovich Ridge, with its oblique ultraslow-spreading and segmentation, makes this end-member of Spreading Ridge Systems an important and challenging ridge to investigate. The aim of this work is to better understand the lithospheric structure beneath the rare ultraslow-spreading ridges, using as example the Knipovich Ridge along its spreading direction. Ultraslow spreading ridges are characterized by a low melt supply. At spreading rates below 20 mm/y, conductive cooling effectively reduces the mantle temperature and results in less melt produced at larger depths. The Ocean Bottom Seismometer (OBS) data along a refraction/reflection profile (~280 km) crossing the Knipovich Ridge off the western Barents Sea was acquired by use of RV G.O. Sars on July 24 - August 6, 2019. The project partners are University of Bergen, Institute of Geophysics, Polish Academy of Sciences, and Hokkaido University. The seismic energy was emitted every 200 m by an array of air-guns with total volume of 80 l. To receive and record the seismic waves at the seafloor, ocean bottom seismometers were deployed at 12 positions with about 15-km spacing in 2 deployments. All the stations were recovered and correctly recorded data. Seismic energy from airgun shots were obtained up to 50 km from the OBSs. The profile provides information on the seismic crustal structure of the Knipovich Ridge and oceanic and continental crust in the transition zone. This profile is a prolongation of the previously acquired profile AWI-20090200 (Hermann & Jokat 2013) and together allow for the modeling of ~535 km long transect crossing the Knipovich Ridge from the American to the European plate. Seismic record sections were analyzed with 2D trial-and-error forward seismic modeling. This work is supported by the National Science Centre, Poland according to the agreement UMO-2017/25/B/ST10/00488. The cruise was funded by University of Bergen.

 

Hermann, T. and Jokat, W., 2013. Crustal structures of the Boreas Basin and the Knipovich Ridge, North Atlantic. Geophys. J. Int., 193, 1399–1414, doi: 10.1093/gji/ggt048

 

How to cite: Czuba, W., Mjelde, R., Murai, Y., and Janik, T.: Ocean Bottom Seismic Survey in the Knipovich Ridge area, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1879, https://doi.org/10.5194/egusphere-egu23-1879, 2023.

X2.127
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EGU23-13813
Morgane Le Saout, Colin W. Devey, Dominik Palgan, and Thorsten S. Lux

The Reykjanes Ridge is a segment of the slow-spreading Mid-Atlantic Ridge interacting with the Iceland plume. The 900 km long segment consists in “en echelon” axial volcanic ridges. They are typically 3-6 km wide, 20-30 km long, 200-500 m high, and overlap with each other over a distance of, on average, 1/3 of their length.  The Reykjanes ridge AVRs have been the subject of several studies and are the base of numerous models of AVRs evolution. However, most of these studies are based on bathymetry with a resolution > 20 m and sidescan data > 5 m, with no geochemical component. Thus, small temporal variations of the accretionary processes, especially changes in eruptive activity and magma composition, are still not well constrained. We here retrace the development of AVRs using high-resolution data combined with lava flow composition. During the MSM75 expedition in 2018, four AVRs between 62.95ºN and 63.20ºN were mapped at the resolution of 5 m. At the 63.08ºN AVR, bathymetric and backscatter data are combined with side-scan sonar data (with a 50 cm resolution) acquired with an autonomous underwater vehicle (AUV Abyss from GEOMAR) and near-bottom video from six remotely operated vehicle dives (ROV Phoca from GEOMAR) to: 1) delineate individual lava flows and tectonic structures, 2) determine flow morphologies (i.e., lobate flows, hummocky flows, hummocky ridges, seamounts), 3) locate extrusion sources, and 4) determine the chronology of the geological events. In addition, the composition of samples collected via ROV and wax corer is used to determine the geochemical evolution of the AVR. Around 200 flow units with distinct morphologies and stages of sedimentation were delineated. Our study reveals that major changes in the flow morphology at 63.08ºN is correlated with changes in flow composition. The AVR development appears to have initiated with the emplacement of seamounts aligned along an eruptive fissure. This was followed by a period of relatively high-extrusion rate / low viscosity eruptions leading to the emplacement of lobate flows. A decrease in extrusion rate and/or increase in viscosity results in the transition from lobate to hummocky morphology. In the last stage, the volcanic activity focuses along numerous narrow hummocky ridges. The similarity of the morphology distribution on several neighboring AVRs in this region indicates comparable evolutions.

How to cite: Le Saout, M., Devey, C. W., Palgan, D., and Lux, T. S.: Morphological and geochemical evolution of the eruptive activity along axial volcanic ridges in the Northern section of the Reykjanes ridge., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13813, https://doi.org/10.5194/egusphere-egu23-13813, 2023.

X2.128
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EGU23-7479
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Highlight
Jean-Arthur Olive, Göran Ekström, W. Roger Buck, Zhonglan Liu, Javier Escartín, and Manon Bickert

Mid-ocean ridges are quintessential sites of extensional deformation, where large-magnitude compressional seismicity is rare and typically confined to transpressional ridge-transform intersections. Here we report on a recent, unusual seismic sequence that included 12 thrust faulting events with magnitudes up to 6, ~25 km off-axis on both sides of the Mid-Atlantic Ridge (MAR) at 54ºN. These compressional events were preceded by a rapidly-migrating swarm of extensional on-axis earthquakes with M≥4.2. We relocated a total of 124 earthquakes and calculated their focal mechanisms using a surface wave-based method. We then modeled the stress state of the ridge flanks to construct a mechanically-consistent interpretation of the sequence, and discuss its significance in terms of seafloor spreading processes.

The sequence started on September 26th, 2022 at 6:07 UTC with a M4.8 normal faulting earthquake at 54º01’N on the Northern MAR, ~125 km north of the Charlie-Gibbs fracture zone. Over 80 normal faulting earthquakes (4.5≤M≤5.8) occurred over the next 3.5 days, with locations steadily migrating southward at ~0.6 km/hr. Earthquake locations form a narrow band that closely follows the axial valley of the symmetric, abyssal hill-bearing 53º30N segment, which is bound by non-transform offsets both to the north and south. Extensional seismicity continued in this band for ~27 more days without a clear propagation pattern. 80 hours into the earthquake swarm, a magnitude-5.7 thrust earthquake occurred ~25 km east of the extensional band. Between September 29, 2022 and January 4, 2023, 11 more thrust events occurred on N-S striking planes east and west of the axis, outlining two narrow bands ~25 km away from the neovolcanic zone. Some of these events seem well aligned with off-axis normal fault scarps, suggesting a possible reactivation of these faults on both flanks.

To better understand this remarkably symmetric pattern of off-axis compression, we model the absolute stress state of the ridge flanks, and the relative stress changes imparted by the on-axis extensional event. 2-D visco-elasto-plastic simulations of slow mid-ocean ridges show that unbending of the lithosphere as it moves out of the axial valley imparts horizontal compression in the cross-axis direction within ~10 to ~40 km away from the ridge axis, and down to ~3 km below seafloor. While this deviatoric compression can reach the brittle yield stress, the associated strain rates are so low that a seismic manifestation of this phenomenon should be extremely rare. On the other hand, the on-axis intrusion of a vertical dike up to a depth of ~5 km below seafloor can put the shallow axis in tension while imparting excess compression on the shallow lithosphere ~25 km off-axis on both sides. Our preferred interpretation is therefore that the extensional swarm represents the southward migration of a blind dike within the neovolcanic axis, which drove both ridge shoulders to compressional failure. Off-axis shortening may thus be an integral component of seafloor spreading that usually operates aseismically, but can be highlighted by certain types of on-axis intrusion events.

How to cite: Olive, J.-A., Ekström, G., Buck, W. R., Liu, Z., Escartín, J., and Bickert, M.: Off-axis compression triggered by a seafloor spreading event on the Northern Mid-Atlantic Ridge, 54ºN, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7479, https://doi.org/10.5194/egusphere-egu23-7479, 2023.

X2.129
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EGU23-3562
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ECS
Anouk Beniest, Katharina Unger Moreno, Lizette Dedecker, Bente Schriever, and Thor Hansteen

The Oceanographer Transform Fault is an oceanic transform fault that offsets a segment of the Atlantic Mid-Oceanic Ridge (MOR), southwest of the Azores. We investigate how deformation is accommodated along an active transform through the interpretation of fault patterns and geomorphological features on high resolution bathymetry and a petrological and kinematic analysis of thin sections.

The bathymetric interpretation yielded six different domains which consisted of 1) the main transform zone with E-W running strike-slip faults, 2) the NNE-SSW oriented MOR valley, 3) the abyssal domain hosting NNE-SSW oriented normal faults that bound the abyssal hills, 4) the abyssal domain hosting NE-SW oriented faults, oriented obliquely to the mid-oceanic ridge and the main transform valley, 5) a volcano- and lava flow rich domain and 6) a shallow domain with corrugations oriented perpendicular to the MOR with little volcanic cover.

The thin section analysis reveals a complete ophiolitic sequence, including serpentinized peridotite, gabbro and basalt with varying degrees of alteration. Samples retrieved from depths >3500 m show that deformation occurs mainly in the ductile domain through bulging and sub-grain rotation of plagioclase, lamellar feldspar formation (in gabbro), shearing and recrystallisation of gabbro and serpentinization of peridotite. Brittle deformation manifests itself through fracturing of crystals, displacement of plagioclase sub-crystal domains and veining. Especially gabbroic samples show a decrease in serpentinized veins with decreasing depth. Basalts are found only at shallow depth, seemingly covering gabbro, appearing not to be affected by deformation at all, only occasional cracks filled with pristine calcite are observed.

The combination of geomorphological features identified on high-resolution bathymetry maps and the petrological and kinematic analysis of thin sections showed that deformation along the transform fault differs from the deformation that happens at the MOR. Deformation at the MOR is characterized by 1) axis-parallel normal faulting, pulses of volcanism, resulting in elongated ridges and volcanic cones on the ocean floor and the formation of dykes under magma-rich circumstances, and core complex exhumation during magma-starved periods that occurred between 1.8 – 4.2 Ma and around 7.5 Ma along the southwestern MOR segment of the OTF and 2) heavily sheared zones that extend obliquely from the MOR-transform intersection into the adjacent older plate. Deformation at the transform fault is accommodated through serpentinization at depths deeper than 3000 m, leading to pop-up structures in the main transform zone and causing fracturing in the overlying gabbro, allowing hydrothermal fluids to heavily alter deeper rocks and migrate to shallower depths with decreasing alteration of the oceanic crust with decreasing depth.

We hypothesize that the transform fault itself at depth accommodates stresses to a large extent via serpentinization processes in response to strike-slip tectonic activity in a very narrow band in the active, deepest part of the main transform zone. Deformation patterns other than serpentinization and serpentinite veining that are observed in rock samples along the transform fault are the result of earlier tectonic activity that took place during or shortly after the formation of the rock at the MOR.

 

How to cite: Beniest, A., Unger Moreno, K., Dedecker, L., Schriever, B., and Hansteen, T.: Deformation along the Oceanographer Transform Fault from fault mapping and thin section analysis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3562, https://doi.org/10.5194/egusphere-egu23-3562, 2023.

X2.130
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EGU23-4327
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ECS
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Szu-Ying Lai, Gaye Bayrakci, Bramley Murton, and Tim Minshull

The Trans-Atlantic Geotraverse (TAG) segment at 26°N on the Mid Atlantic Ridge (MAR) is notable for hosting hydrothermal mounds and seafloor massive sulphide deposits. At the slow-spreading MAR, detachment faulting plays an important role in controlling the seafloor morphology. In this study, we investigate the seismic velocity in the upper crust at a finer scale than previously possible, and its relationship to fault structures.

We used short-offset ocean bottom hydrophone (OBH) data collected during the Meteor 127 cruise in 2016. The survey was designed mainly to study the hydrothermal mounds. We chose a NW-SE trending, 11-km long wide-angle seismic profile that crosses a detachment breakaway identified from AUV bathymetry and seismic reflection profiles. The source was a G-gun array of 760 c. inch towed at 6 m depth. The shot spacing was 12 s (15-20 m) with four OBHs at 1.3 km spacing.

A two-dimensional P-wave velocity model was generated by first-arrival travel-time tomography using the TOMO2D code. We used as our starting model the average 1D velocity depth function of a slice along our profile through Zhao et al’ s (2012) three-dimensional velocity model. Our final tomographic model reveals crustal velocities from 3.4 km/s to 5 km/s for the upper 600 m below seabed. Most of the profile lies beneath the eastern valley wall, where a corrugated detachment surface crops out. Beneath the detachment surface in our profile, we observed an increased velocity of 6.5 km/s at 1.5 km below seabed. Our velocity model suggests that the west-dipping normal fault exhumes lower crust of velocity up to 6.5 km/s.

How to cite: Lai, S.-Y., Bayrakci, G., Murton, B., and Minshull, T.: High-resolution upper crustal structure from OBH data at the TAG Hydrothermal Field, 26°N on the Mid-Atlantic Ridge, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4327, https://doi.org/10.5194/egusphere-egu23-4327, 2023.

X2.131
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EGU23-2767
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Zhikui Guo, Lars Rüpke, and Chunhui Tao

The Earth System appears increasingly interconnected and hydrothermal discharge at back smoker vent sites is not only visually appealing, it also sustains unique ecosystems, generates large polymetallic sulfide deposits, and modulates ocean biogeochemical cycles. At slow spreading ridges, fault zones seem to provide stable preferential fluid pathways resulting in the formation of the ocean’s largest sulfide deposits. The TAG hydrothermal mound at 26°N on the Mid-Atlantic Ridge (MAR) is a typical example located on the hanging wall of a detachment fault. It has formed through distinct phases of high-temperature fluid discharge lasting 10s to 100s of years throughout at least the last 50,000 years and is one of the largest sulfide accumulations on the MAR. Yet, the mechanisms that control the episodic behavior, keep the fluid pathways intact, and sustain the observed high heat fluxes of possibly up to 1800 MW remain poorly understood. Previous concepts involved long-distance channelized high-temperature fluid upflow along the detachment but that circulation mode is thermodynamically unfavorable and incompatible with TAG's high discharge fluxes. Here, based on the joint interpretation of hydrothermal flow observations and 3-D flow modeling, we show that the TAG system can be explained by episodic magmatic intrusions into the footwall of a highly permeable detachment surface. These intrusions drive episodes of hydrothermal activity with sub-vertical discharge and recharge along the detachment. This revised flow regime reconciles problematic aspects of previously inferred circulation patterns and allows to identify the prerequisites for generating substantive seafloor mineral systems.

How to cite: Guo, Z., Rüpke, L., and Tao, C.: Detachment-parallel recharge explains high discharge fluxes at the TAG hydrothermal field-Insights from 3D numerical simulation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2767, https://doi.org/10.5194/egusphere-egu23-2767, 2023.

X2.132
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EGU23-14452
Ewan Pelleter, Cecile Cathalot, Stéphanie Dupré, Mathieu Rospabe, Thomas Giunta, Boissier Audrey, Sandrine Cheron, Mickael Rovere, Robin Bonnet, Paco Ferrand, Laetitia Leroy, Yoan Germain, Vivien Guyader, Jean-Pierre Donval, and Yves Fouquet and the Ewan Pelleter

Since 1977 and the discovery of the first high temperature (HT) hydrothermal vent, more than 300 sites are known (about 600 including inferred ones). Among these hydrothermal sites, the talc-rich deposit is the most recent class of hydrothermal system discovered on the seafloor [1]. Only three talc-rich deposits have been described so far: (i) the active Von Damm Vent Field (VDVF), (ii) the inactive St Paul’s and (iii) Conrad fracture zones deposits [2]. These hydrothermal sites are associated with lower crustal rocks and/or serpentinized peridotites and might be widespread at slow or ultraslow spreading ridge. However, no clear spatial or temporal relationship of this new class of hydrothermal system and the “black smoker”-like system has been highlighted.

 During the HERMINE (March-April 2017) and HERMINE2 (July-August 2022) cruises [3], [4], two hydrothermal areas with talc-rich deposits have been discovered during Nautile HOV dives. The first one (23°N) is an inactive hydrothermal area located 28km northwest of the Snake Pit vent field (25km west of the axial rift). At least two deposits have been observed: (i) a talc-silica deposit and (ii) a fully oxidized SMS-type deposit characterized by copper concentrations up to 3.3wt.%. The second hydrothermal area (26°N) is composed of one large and weakly-active deposit composed of silica-sulfides rocks and at least two small talc-silica deposits. To our knowledge, this is the first time that such a spatial relationship has been described between these two classes of deposits. The preliminary results on these newly discovered hydrothermal field will be presented here.

 

[1] Hodgkinson et al. (2015) Nat.. Commun 6:10150

doi: 10.1038/ncomms10150 .

[2] D’Orazio et al. (2004) Eur. J. Mineral. 16, 73-83

[3] Fouquet and Pelleter (2017), https://doi.org/10.17600/17000200

[4] Pelleter and Cathalot (2022),

https://doi.org/10.17600/18001851

How to cite: Pelleter, E., Cathalot, C., Dupré, S., Rospabe, M., Giunta, T., Audrey, B., Cheron, S., Rovere, M., Bonnet, R., Ferrand, P., Leroy, L., Germain, Y., Guyader, V., Donval, J.-P., and Fouquet, Y. and the Ewan Pelleter: Spatial association between talc-rich mineralization and sulfide-bearing deposits in a newly discovered inactive and weakly actie fields (Mid-Atlantic Ridge), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14452, https://doi.org/10.5194/egusphere-egu23-14452, 2023.

X2.133
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EGU23-13265
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Highlight
Javier Escartin and Muriel Andreani and the Arc-en-Sub Science Party

The ultramafic Rainbow Massif hosts the high-temperature (HT) Rainbow hydrothermal site, venting H2, CH4 and Fe-rich fluids that support unique macro- and microbial ecosystems. This Massif also sustained low-temperature (LT) hydrothermal circulation associated to fossil bivalve communities, identified at two sites, Clamstone and Ghost City, with 14C and U-Th dates of 25.5 and 110 kyrs, respectively. Furthermore, the Massif is also underlain by seismic reflectors interpreted as stacked melt lenses, the potential heat source for fossil and active hydrothermal outflows. To understand the diversity, controls, and history of ultramafic-related hydrothermal circulation, and how these different systems are sustained over time, the Arc-en-Sub cruise (May 2022) conducted (1) a compliance experiment to determine if deep-seated reflectors are melt-bearing at depth, (2) extensive bathymetric mapping (70 km2) and magnetic surveying with the Autonomous Underwater Vehicle (AUV) IdefX, and (3) extensive geological observations, sampling, and seafloor imaging (3D and photomosaicing) with the Remotely Operated Vehicle (ROV) Victor, along ~100 km of bottom tracks.

Preliminary cruise results reveal corrugated detachment fault surfaces along its western flank, and confirm that the massif is associated with a detachment system rooting westwards, along the S-AMAR ridge segment. The AUV microbathymetry also shows a complex tectonic history of oblique high-angle normal faulting, small-scale detachment faulting, and late strike-slip deformation, with temporal changes yet to be analyzed.

ROV observations and sampling confirmed the dominance of ultramafic rocks in the massif substrate, and revealed previously unknown hydrothermal sites, both active and fossil. First, in addition to Rainbow, we have identified several active sites of a new type, with LT fluids venting at temperatures from a few degrees above ambient seawater, and up to 70°C. This discovery significantly extends the style and areal exposures of present-day activity well beyond the HT Rainbow hydrothermal field (> 10 km2). Second, we have identified numerous fossil carbonate and sulfide hydrothermal chimneys at various locations on the massif that are sometimes in close spatial association, suggesting a temporal evolution of local hydrothermal style. Third, fossil bivalve communities are found over much broader areas than previously described (hundreds of m2), extending along the summit of the Massif and its western flank, demonstrating an extensive, and pervasive diffuse flow in the past. Dating of these sites within a detailed structural framework will constrain the timing and duration of these different hydrothermal events to better evaluate their relationships and their links to the magmatic and structural evolution of the massif. These preliminary cruise results already show complex spatio-temporal dynamics of fluid flow, resulting in a far more varied and widespread hydrothermal activity than expected on ultramafic-hosted environment along mid-ocean ridges. These results also provoke further consideration of the impact of ultramafic hydrothermal systems on thermal and chemical ocean-lithosphere exchanges.

 

How to cite: Escartin, J. and Andreani, M. and the Arc-en-Sub Science Party: Diversity and dynamics of ultramafic-hosted hydrothermal activity at mid-ocean ridges : first results from the Arc-en-Sub oceanographic cruise, Rainbow Massif, 36°14’N MAR, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13265, https://doi.org/10.5194/egusphere-egu23-13265, 2023.

X2.134
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EGU23-5648
Marcia Maia, Anne Briais, Lorenzo Petracchini, Marco Cuffaro, Marco Ligi, Daniele Brunelli, Lea Grenet, and Cédric Hamelin

We studied the east intersection between the Romanche transform fault (TF) and the Mid-Atlantic ridge using bathymetry and gravity anomalies, to investigate the temporal evolution of the ultra-cold ridge-transform intersection. Our results reveal a complex ridge axis, with evidence of a significant decrease in the along-axis melt supply towards the RTI but also since ~10 Ma.

Over a 100 km distance south of the RTI, the ridge axis is formed by three spreading segments offset by large non-transform discontinuities. Large detachment faults mark the present-day spreading style at the RTI, while magma supply increases away from the Romanche intersection. Axial and near-axis fault patterns reveal a marked obliquity, especially in the north and center of the study area.In lithosphere older than 10 Ma, the ridge axis appears to form a single spreading segment between the Romanche and Chain TFs, perpendicular to the spreading direction, with relatively regular abyssal hills. From around 10 to 3 Ma, oceanic core complexes (OCCs) developed in the northern part of the ridge axis south of the Romanche TF.  The complexity of the ridge axis appears to have increased in the last 3 Ma, with ridge obliquity accompanying axial instabilities and ridge jumps.  At least three eastward ridge relocations were identified immediately south of the Romanche TF, rupturing a series of OCCs located in the African plate, east of the ridge axis. This pattern could reflect a progressive decrease in the melt supply, in particular since 3-5 Ma. This may be related to a significant reduction of the ridge spreading rate as seen from kinematic models which allowed the cooling effect of the large offset Romanche TF to dominate the spreading processes in the area.

How to cite: Maia, M., Briais, A., Petracchini, L., Cuffaro, M., Ligi, M., Brunelli, D., Grenet, L., and Hamelin, C.: Temporal variation in spreading processes at the Eastern Romanche-Mid-Atlantic Ridge intersection, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5648, https://doi.org/10.5194/egusphere-egu23-5648, 2023.

X2.135
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EGU23-11239
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ECS
Rim Jbara, Anne Briais, Etienne Ruellan, Georges Ceuleneer, and Marcia Maia

The George V and Tasman Transform Fault Systems (TFS) are major, right-stepping offsets of the South-East Indian Ridge between 140°E and 148°E. The George V TFS (~140°E) has an offset of about 300 km, and the Tasman TFS (~148°E) an offset of about 600 km. These TFS have multiple shear zones with intra-transform ridge segments (ITRS), mostly unmapped yet. We present the results of the analysis of geophysical and petrological data collected during the STORM cruise (South Tasmania Ocean Ridge and Mantle), completed with global data sets including satellite-derived gravity and bathymetry, and earthquake distribution. The swath bathymetry data cover some parts of the shear zones and only a few of ITRSs. They reveal a complex interaction between tectonic processes at the plate boundary and near-axis volcanic activity along and across the transform faults. In both the George V and Tasman TFS the western ITRS are shallower than the eastern ones, and they appear to receive a lot more magma supply. These western ITRS display off-axis volcanism observed on swath bathymetry or suspected from free-air gravity anomaly highs. In both TFS also, the western shear zone consists of two segments separated by a tectonic massif which we interpret to represent a push-up resulting from transpression along the transform. The mechanism involved in generating the transpression is a lengthening of the western ITRS to the west due to its high magma supply, leading to an overlap between the ITRS and the ridge segment immediately to the west of the TFS, that is in a mechanism similar to the processes currently uplifting the mylonitic massif along the St. Paul TF in the Equatorial Atlantic. The bathymetric and backscatter maps of the western George V TFS also reveal a series of recent off-axis oblique volcanic ridges. Rocks dredged on one of these ridges consist of picrites (i.e. basalts rich in olivine phenocrysts). These observations suggest that both TFS are not magma starved like many mid-ocean ridge transforms, but are the locus of significant primitive melt supply. Such an unexpected production of high-Mg melt might be related to the presence of a mantle thermal anomaly beneath the easternmost SEIR, the result of regional extension following clockwise rotations of the spreading direction, and/or to a western flow of mantle across the TFS. Some of the ITRS actually appeared after changes in the Australia-Antarctic plate motion.

How to cite: Jbara, R., Briais, A., Ruellan, E., Ceuleneer, G., and Maia, M.: Characteristics of the George V and Tasman Transform Fault systems, South-East Indian Ridge, and implications for mantle dynamics., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11239, https://doi.org/10.5194/egusphere-egu23-11239, 2023.

X2.136
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EGU23-13725
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ECS
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Antoine Demont, Jean-Arthur Olive, and Mathilde Cannat

Large-offset detachment faults are common at slow-spreading mid-ocean ridges (MORs). They are typically thought to form in ridge portions that receive a moderate supply of magma. However, they are also found along certain sections of ultraslow-spreading MORs that are largely amagmatic, and feature unusually cold and thick (>15 km) brittle lithosphere. Here we combine geological observations and numerical simulations to assess how these unusual conditions enable and modulate the growth of detachments.

We simulate amagmatic seafloor spreading using 2-D thermo-mechanical models with self-consistent thermal evolution. The brittle lithosphere is modeled as a Mohr-Coulomb elasto-plastic material whose friction decreases with accumulated plastic strain. Ductile deformation is parameterized through experimentally-derived olivine flow laws.

We first investigate how the strength contrast between the fault zone and surrounding lithosphere affects tectonic styles. Geological observations suggest fault zones have lower effective friction coefficients due to serpentinization and fluid circulation.  Evidence for grain size reduction in  ultramafic rocks also suggests additional ductile weakening. In our simulations, varying the strength contrast between faults and lithosphere leads to 3 regimes:  (1) a stable detachment that migrates toward its hanging wall; (2) the sequential growth of horsts bound by two active antithetic faults; and (3) “flip-flopping” detachments that cross-cut each other, comparable to those documented in the natural case. A greater contrast in friction and/or cohesion favors the stable detachment mode, which is consistent with previous studies.

We next focus on the specific effect of a strong, viscous lower lithosphere on brittle deformation in the upper lithosphere. We do so by comparing simulations that use dry olivine flow laws for rocks hotter than ~700ºC with models in which the brittle lithosphere sharply transitions into a low-viscosity asthenosphere. We find that a strong lower lithosphere favors more distributed faulting and shifts the transition to the stable detachment regime to greater strength contrasts.

We also investigate the impact of pervasive fluid circulation in the shallow axial lithosphere, which manifests as active hydrothermal sites. We parameterize its mechanical and thermal effect, i.e., reducing the effective normal stress through a hydrostatic fluid pressure and efficiently cooling young lithosphere. While the latter strongly modulates the depth to the brittle-ductile transition, we find that the former has small effect on tectonic styles, akin to a slight weakening of unfaulted lithosphere.

Finally,  extensive mass wasting is also documented at mid-ocean ridge detachments, but its potential effect on tectonics remains poorly known. We implement diffusive erosion of the model's free surface, which promotes a transition from the stable to flip-flopping detachment regime. This is possibly due to a modulation of topographic stresses.

Overall, because of the delocalizing effect of a strong ductile lithosphere, the growth of detachments at cold, amagmatic MOR sections requires some degree of rheological weakening, both in the brittle and ductile domains. We find, however, that even moderate frictional weakening (e.g., a friction coefficient of 0.4) which can be attributed to serpentinization of the fault zone, can be sufficient to promote large-offset faulting, a process that may be aided by mass redistribution at the seafloor.

How to cite: Demont, A., Olive, J.-A., and Cannat, M.: Detachment fault growth modulated by brittle softening and ductile flow in amagmatic (ultra)slow-spread oceanic lithosphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13725, https://doi.org/10.5194/egusphere-egu23-13725, 2023.

X2.137
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EGU23-4289
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Highlight
Ingo Grevemeyer, Timothy J. Henstock, Anke Dannowski, Milena Marjanovic, Helene-Sophie Hilbert, Yuhan Li, and Daman A. H. Teagle

Our view on the structure of oceanic crust is largely based the interpretation of seismic refraction and wide-angle experiments, revealing that the upper basaltic crust (layer 2) is a region of strong velocity gradients. In contrast, the lower gabbroic crust (layer 3) is relatively homogeneous, although it generally displays a gentle increase in velocity with depth. Furthermore, the upper crust has been sub-divided into layer 2A, composed of extruded basalts, and layer 2B, formed by basaltic sheeted dikes. Site 1256, drilled during the Ocean Drilling Program (ODP) into the upper crust and later extended into the uppermost gabbroic crust during the Integrated Ocean Drilling Program (IODP), is among the deepest drill sites sampling intact oceanic crust. It is the only site world-wide that crossed the entire basaltic upper crust, reaching plutonic rocks at ~1.35 km below the top of the basement, recovering 150 m of dominantly gabbroic rocks at the base of the hole. Three campaigns of down-hole logging at hole 1256D provided a unique set of high-resolution sonic-log velocities of seismic layer 2 and from the uppermost top of seismic layer 3. However, Hole 1256D was drilled at a site with rather limited seismic data coverage, especially lacking seismic refraction and wide-angle profiling. During a seismic survey of the RRS JAMES COOK in the Guatemala Basin in December of 2022, a seismic profile with 12 Ocean-Bottom-Seismometers spaced at 7 km intervals, receiving signals from a tuned airgun array of 4500 cubic-inches shot at 150 m spacing was collected. The data provide excellent seismic records to derive a detailed sound-velocity model of the oceanic crust at the drill site from tomographic travel time inversion of first arrivals (Pg, Pn) and a prominent wide-angle reflection from the crust-mantle boundary (PmP) or seismic Moho. The results show that the seismic structure along the 115 km long line is extremely homogeneous. The velocity-depth profile from tomography further provides an excellent low-frequency match of the down-hole logging observations, supporting that modern seismic data are a powerful remote sensing tool to study the oceanic crust and lithosphere. An interesting observation is that the thickness of the oceanic crust at Site 1256 is extremely thin at only 4.6 to 5.1 km, compared to a global average thickness of about 6 km. This appears to be a regional feature supported by another seismic profile about 150 km north-eastwards. The thin crust agrees with a weak seismic event at ~6.8 s two-way travel time (twtt), i.e., ~1.6 s twtt below basement obtained from re-processing 6-km-long streamer data from the ODP pre-site survey at Site 1256.

 

How to cite: Grevemeyer, I., Henstock, T. J., Dannowski, A., Marjanovic, M., Hilbert, H.-S., Li, Y., and Teagle, D. A. H.: Oceanic crustal structure at ODP Site 1256 from seismic wide-angle tomography and down-hole logging, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4289, https://doi.org/10.5194/egusphere-egu23-4289, 2023.

X2.138
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EGU23-7119
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ECS
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Yu Ren, Dietrich Lange, and Ingo Grevemeyer

Plate tectonics defines oceanic transform faults as long-lived tectonics features. In the Pacific Ocean their traces, called fracture zones, can easily be identified as several thousands of kilometer-long features in bathymetric and gravity field data. However, today none of the fracture zones in the North Pacific are directly linked to any mid-ocean ridge-transform fault. This feature is related to the subduction of the Farallon spreading center and a major change in the direction of plate motion several millions of years ago. Consequently, ridge segmentation is adjusting to a new tectonic framework. The Blanco transform fault system (BTFS) in the northwest off the coast of Oregon is one of the newly evolving transform faults. It is highly segmented and shows strong similarities with other segmented oceanic transform systems, such as the Siqueiros in the East Pacific Rise, which developed from a pre-existing transform fault subjected to a series of extensional events due to a documented change in spreading direction. However, plate tectonic reconstructions suggested that the BTFS developed from at least two large ridge offsets rather than a single transform fault, emerging from a series of ridge propagation events after the plate reorientation at ~5 Ma.
We used one year of ocean-bottom-seismometer data from the Blanco Transform OBS Experiment (2012-2013) and high-resolution multibeam bathymetry, aeromagnetic, and gravity datasets to study the seismotectonic behavior and tectonic evolution of the BTFS. Interestingly, all available datasets provide no evidence for the existence of either transform faults or fracture zones around the BTFS before 2 Ma, supporting that there were no pre-existing transform faults before the initiation of the BTFS. Therefore, we suggest the BTFS developed from two broad transfer zones instead of pre-existing transform faults. We present seismicity and focal mechanisms for stronger manually-picked events.  Furthermore, the seismic data were picked with a phase picker learned with a large OBS training dataset. The resulting seismicity of ~8,000 events reveals the present-day deformation of the fault system with very high spatial resolution, and supports substantial along-strike variations, indicating different slip modes in the eastern and western BTFS. Seismic slip vectors suggest that the eastern BTFS is a mature transform fault system accommodating the plate motion. At the same time, the western BTFS is immature as its re-organization is still active. The BTFS acts as a natural laboratory to yield processes governing the development of transform faults away from continental rift zones.

How to cite: Ren, Y., Lange, D., and Grevemeyer, I.: Immature transform plate boundaries in the Northeast Pacific: Constraints from ocean bottom seismology, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7119, https://doi.org/10.5194/egusphere-egu23-7119, 2023.

X2.139
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EGU23-7653
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ECS
Clément Estève, Yajing Liu, Gong Jianhua, and Wenyuan Fan

Fast-slipping mid-ocean ridge transform faults are characterized by quasi-periodic seismic cycles with typical inter-event times of 5 to 8 years. In particular, the Gofar transform fault (GTF) of the East Pacific Rise, generates a MW ~ 6 earthquake every 5 to 6 years on short (~20 km) along-strike segments separated by a barrier zone. Therefore, the GTF presents the opportunity to investigate the relation between fault structure and material properties of this fault to earthquake processes. Here, we perform a joint inversion of P- and S-wave arrival times from local earthquakes to develop three-dimensional seismic velocity models (VP, VS and VP/VS) of the easternmost and westernmost segments (G1 and G3, respectively). The velocity models reveal that G3 is characterized by a more heterogeneous fault zone velocity structure compared to G1. Sharp velocity contrasts are observed along G3 interpreted to reflect along-strike variations in material properties. G1 is characterized by large low-velocity anomaly extending through the entire oceanic crust with subtle along-strike variations. The 2020 Mw 6.1 earthquake occurred within a low VP, low VS and high VP/VS patch along G1 whereas the 2008 Mw 6 earthquake occurred on sharp VP, VS and VP/VS contrast. We also note similarities between the two fault segments. In particular, rupture barrier zones are characterized by a high rate of seismicity and a rapid decrease following the mainshock. We also note the occurrence of deep seismicity in low VP/VS patches beneath the rupture barrier zones, which may indicate sea-water infiltration at 10 to 14 km depth below sea level.

 

How to cite: Estève, C., Liu, Y., Jianhua, G., and Fan, W.: Earthquake relocations and three-dimensional VP, VS and VP/VS along the fast-slipping Gofar oceanic transform fault, East Pacific Rise., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7653, https://doi.org/10.5194/egusphere-egu23-7653, 2023.

X2.140
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EGU23-8330
Meir Abelson, Lior Kamahaji, Ron Shaar, and Amotz Agnon

The domal structure in the core of the Troodos ophiolite exposes lower crustal (gabbro suite) and mantle rocks (ultramafic province). This structure is part of a fossil ridge-transform intersection (RTI), where an extinct spreading axis meets the fossil oceanic transform, namely the Arakapas transform. A major feature in the RTI system is the Troodos Forest-Amiandos Fault (TAF), an off-axis and axis parallel fault that was active during the Cretaceous seafloor spreading. Here we investigate the deformation across the TAF by measuring paleomagnetic vectors from 34 sites in the gabbro suite around the domal ultramafic core. Special emphasize was along an E-W transect that crosses the TAF south of the sheeted dike complex and north of the ultramafic province. We also compiled dike dips along an E-W strip (6 km wide) north of the gabbro suite. All results were compared to previous paleomagnetic studies from the sheeted dikes and the gabbro suite. Accordingly, we have found that rotations in the gabbro are very similar to those in the sheeted dikes, suggesting coupling of the upper and the lower oceanic crust during axial deformation of seafloor spreading. All rotation axes were horizontal and parallel to the dike strikes, i.e., parallel to the extinct spreading axis. Rotations increase gradually towards the TAF from both sides, eastward in the footwall and westward in the hanging wall. The most plausible scenario is an upward and downward deflection in the footwall and the hanging wall, respectively, similarly described theoretically for the early stages of detachment development. The orientations of the rotation axes of all paleomagnetic vectors indicate spreading-related deformation. This suggests that the relative uplift of the deep-seated rocks was by the development of a young detachment during seafloor spreading rather than serpentinite diapirism. The detachment occurrence in the outside-corner is explained here by the shift from orthogonal to curved axis, inferred from sheeted dike orientations.

How to cite: Abelson, M., Kamahaji, L., Shaar, R., and Agnon, A.: Lithosphere deflection on a juvenile oceanic detachment during seafloor spreading promoted the exposure of the mantle rocks of the Troodos ophiolite – inferences from gabbro paleomagnetism, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8330, https://doi.org/10.5194/egusphere-egu23-8330, 2023.

X2.141
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EGU23-13869
Nico Augustin, Morgane Le Saout, Cora K. Schiebener, and Froukje M. van der Zwan

The mid-ocean rift in the Red Sea is recently regaining attention in the geosciences due to the possibility of investigating this young ocean in more detail than ever by state-of-the-art methods and modern deep-sea instrumentation. During the first AUV surveys of the Red Sea rift in Spring 2022, we collected multibeam bathymetry, backscatter, sub-bottom, and water column data over a 9 km long ridge segment in the Hadarba Deep between 22.49°N and 22.56°N to investigate the volcano-tectonic processes of this mid-ocean ridge. This area's total spreading rate of about 12 mm per year is defined as ultra-slow spreading. The high-resolution hydroacoustic data of the used Kongsberg Hugin Superior AUV (operated by Fugro) revealed more than 100 individual lava flows with different stages of sedimentation. The oldest lava flows are buried under 3-4 m of sediment, indicating ages of up to 28 ka. A dome volcano with a 2.5 km diameter and an average height of 300 m dominates the mapped area but has been inactive for at least ~8.4 ka. Several younger lava flows show recent episodes of volcanism along the rift axis. However, their sediment cover is below the vertical sub-bottom-profiler resolution of about 10 cm and thus might be only a few hundred years old or younger. We will present our geomorphological maps, analyses, and statistics that reveal a moderately faulted, ultra-slow spreading MOR segment in the Red Sea with a surprisingly large amount of magmatic extension and show implications for the formation history of this ridge segment.

How to cite: Augustin, N., Le Saout, M., Schiebener, C. K., and van der Zwan, F. M.: High-resolution geomorphological studies of a Red Sea Rift segment in Hadarba Deep, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13869, https://doi.org/10.5194/egusphere-egu23-13869, 2023.